In recent years, as the information amount required for devices such as information devices and video-audio devices has been increased, there has been increased a demand for information recording media such as an optical disc easily accessible to data, capable of storing a large amount of data, and advantageously used in miniaturizing the device. High density of recording information is realized in such an optical disc. For instance, as means for realizing a high-density optical disc, there have been proposed a single-layer optical recording medium having a recording capacity of about 25 GB, and a two-layer optical recording medium having a recording capacity of about 50 GB. In these optical recording media, information is recorded or reproduced by a reproduction head using laser light having a wavelength of about 400 nm, and a collecting lens for converging the laser light and having a numerical aperture (hereinafter, also called as “NA”) of 0.85. Further, there has also been proposed a multilayer information recording medium having three or more information recording layers.
FIG. 19 is a cross-sectional view of a conventional multilayer information recording medium. The multilayer information recording medium shown in FIG. 19 is constituted of a signal substrate 201 having a transferred concave-convex information surface with pits or guide grooves on one surface thereof, a first film layer 202 disposed on the concave-convex information surface of the signal substrate 201, an intermediate layer 203 having a transferred concave-convex information surface with pits or guide grooves on a surface thereof opposite to the surface adhered to the first film layer 202, a second film layer 204 disposed on the concave-convex information surface of the intermediate layer 203, a transparent substrate 206 disposed opposite to the intermediate layer 203, and a transparent layer 205 adapted to adhere the second film layer 204 and the transparent substrate 206 to each other.
Pits or guide grooves are transferred and formed on one surface of the signal substrate 201 by e.g. injection compression molding using a stamper. An information recording layer is formed by forming a film layer on an information surface, as described above. The thickness of the signal substrate 201 is about 1.1 mm. The first film layer 202 and the second film layer 204 include a recording film and a reflection film. The recording film and the reflection film are formed on the surfaces of the signal substrate 201 and the intermediate layer 203 having the pits or the guide grooves by e.g. sputtering or vapor deposition.
The intermediate layer 203 is formed by a spin coating method using a photo-curable resin. Specifically, a transfer substrate having pits or guide grooves on one surface thereof like the signal substrate 201 is adhered, with an information surface thereof opposed to the signal substrate 201, by way of a photo-curable resin. After the photo-curable resin is photo-cured, the transfer substrate is peeled off from a boundary with the photo-cured resin layer, whereby the intermediate layer 203 is formed.
The transparent substrate 206 is made of a transparent material having transparency with respect to recording light or reproducing light, and has a thickness of about 0.1 mm. The transparent layer 205 is provided to adhere the two substrates 206 and 207 to each other, and is made of an adhesive agent such as a photo-curable resin or a pressure sensitive adhesive agent. The transparent substrate 206 and the transparent layer 205 as a whole may also be called as a cover layer. The cover layer may be formed by curing the transparent layer 205, without adhering the transparent substrate 206. Recording or reproducing information with respect to the multilayer information recording medium having the above construction is performed by irradiating the multilayer information recording medium with recording layer light or reproducing laser light from the side of the transparent substrate 206.
In the multilayer information recording medium having the above construction, it is often the case that the intermediate layers and the cover layer are formed by a spin coating method using e.g. a UV curable resin (see e.g. patent literature 1).
However, in the case where transparent intermediate layers for use in separating adjacent information surfaces from each other, and a cover layer are formed by a spin coating method, small film thickness variations in circumferential direction, and large film thickness variations in radial direction may occur. In particular, there is a problem that film thickness variations are likely to increase resulting from accumulation of film thickness variations in laminating plural information recording layers and plural intermediate layers. Further, in the case where a UV curable resin is coated by a spin coating method, the resin spreads to an outer perimeter of a substrate to be coated. As a result, when the spin rotation is stopped, and the resin is cured by light irradiation, the resin may be swollen on the outer perimeter of the substrate to be coated by surface tension, with the result that the film thickness may be increased.
Because of the film thickness variations, when a signal is recorded or reproduced with respect to the multilayer information recording medium by using a laser, spherical aberration may be generated, which may affect variations of convergence of a beam spot, focus control of collecting a beam spot on an information surface, and tracking control of controlling a beam spot to follow a signal train. Further, in the spin coating method, since control on the conditions for realizing coat thickness uniformity is complicated, and spin coating is performed layer by layer, it is difficult to shorten a tact time.
On the other hand, in the case where the number of information recording layers to be laminated is increased to increase the recording capacity, and plural resin layers are laminated, high thickness precision is required. This is because of the following reason. Specifically, it is necessary to know the positions of the respective layers in the multilayer information recording medium by the main body of a reproducing device in advance in order to reproduce signals from the multilayer information recording medium by the reproducing device. The positional displacement between the respective layers results from a thickness distribution of a resin layer. Further, as the number of resin layers to be laminated is increased, high-precision position information is required. In view of the above, it is necessary to increase the thickness precision of the respective resin layers in the multilayer information recording medium.
Further, if there is a thickness variation inherent to a process of forming a resin layer, the thickness variation is accumulated, each time a resin layer is formed. As a result, the thickness variation from a surface of the multilayer information recording medium to a farthest information recording layer thereof is increased, which may make it difficult to perform focus control, and may deteriorate the signal quality.
FIG. 20 is a diagram showing a signal substrate for a conventional optical disc. FIG. 21 is a cross-sectional view of the signal substrate for the optical disc shown taken along the line 21-21 in FIG. 20. FIG. 22 is a diagram for describing a thickness distribution of a resin layer.
In the case where a resin layer is formed on a signal substrate for an optical disc by screen printing, a thickness distribution may vary depending on the shape of the substrate. For instance, as shown in FIG. 20, a signal substrate 300 for an optical disc is formed with a convex rib 301 on a central part thereof. The rib 301 is provided to prevent the user's difficulty in picking up an optical disc which may be contacted with the optical disc surface with a table or a floor, in the case where the optical disc is placed on a flat surface such as a table or a floor. Forming a protrusion on an inner periphery of an optical disc makes it easy for the user to pick up the optical disc, because there is formed a clearance between the table surface and the optical disc surface.
The rib 301 exhibits the above advantage by protrusion from the optical disc surface. In view of this, the rib 301 protrudes with a larger amount than the thickness of a resin layer to be formed in a substrate. The rib 301, however, may seriously affect in forming a resin layer by screen printing.
The rib 301 shown in FIG. 21 has a height of 0.2 mm from an information surface 302 at a position radially away from the center of the signal substrate 300 for an optical disc by 18 mm to 20 mm. A resin layer of 0.1 mm in thickness is formed on the information surface 302 of the substrate 300. Accordingly, the manufactured optical disc has a protrusion of 0.1 mm in height.
As shown in FIG. 22, in the case where the signal substrate 300 for an optical disc having the rib 301 at the inner periphery thereof is subjected to screen printing, the thickness of a resin layer is increased by a region where a squeegee 401 is contacted with the rib 301. This phenomenon is likely to occur particularly on an inner periphery of the disc near the rib 301.
Accordingly, as shown in FIG. 22, the thickness of a resin layer is increased on an elliptical area 403 in the inner periphery of the signal substrate 300 for an optical disc, as compared with the other area. In FIG. 22, the squeegee 401 slides in the direction shown by the arrow 402.
The thickness distribution of a resin layer varies because the squeegee 401 is made of a soft material such as rubber. FIG. 23 is a diagram showing a squeegee and a signal substrate for an optical disc when the squeegee passes a central part of the signal substrate for an optical disc. FIG. 24 is a cross-sectional view of the squeegee and the signal substrate for an optical disc taken along the line 24-24 in FIG. 23. In FIG. 23, the squeegee 401 slides in the direction shown by the arrow 402.
As shown in FIG. 24, when the squeegee 401 is contacted with the rib 301, the squeegee 401 is pushed upward by the rib 301. As a result, there is formed a clearance 404 between the squeegee 401 and the signal substrate 300 for an optical disc, at a periphery of the rib 301. Since the clearance between the squeegee 401 and the signal substrate 300 for an optical disc at the periphery of the rib 301 is increased, as compared with the other part, the thickness of a resin layer to be formed is increased at the periphery of the rib 301. This phenomenon is serious on the area 403 near the rib 301, whereas the thickness of the resin layer becomes substantially uniform on an area away from the rib 301. Further, the area 403 having a larger resin layer thickness is formed only in a region where the squeegee 401 is contacted with the rib 301.
As described above, a resin layer formed by screen printing has thickness variation along the moving direction of the squeegee that the thickness is small on a disc end, and the thickness is large on a disc inner periphery. As a result, particularly, in the case where plural resin layers of a multilayer information recording medium having plural information recording layers are formed by screen printing, the problem that the thickness variation is increased becomes serious. Specifically, in the case where the areas each having a largest resin layer thickness are overlapped on the disc inner periphery, the thickness variations are accumulated as a whole. If a cover layer is formed on the resin layers in this condition, thickness variation from the disc surface to the farthest information recording layer is increased, which may seriously affect the signal quality in recording or reproducing information with respect to the multilayer information recording medium.